1. Field of the Invention
The present invention relates to a heavy-wall steel having a flange thickness of about 40 mm or more. The invention can be used as a structural member, such as a column or a beam in a high-rise building, and can have an H-shape. This invention more specifically relates to the heavy-wall steel having excellent strength, toughness, weldability and seismic resistance.
2. Description of the Related Art
Hot-rolled gauge H steels are widely used as column members and beam members of buildings. Particularly, SM490, SM520, and SM590 gauge H steels that are standardized as rolled steels for welded structures by JIS G 3106 are frequently used. New buildings are continually being built to a larger scale, and in response, the gauge H steels being used are increasingly thicker and stronger. Presently, there is a demand for gauge H steels having a yield point or yield strength (YS) of 325 MPa or more, or of 355 MPa or more, a yield ratio of 80% or less, and excellent toughness.
However, the strength of ordinary steel is prone to decrease as thickness increases. In fact, it is difficult to provide high YS of 325 MPa or more, or 355 MPa or more, for a heavy-wall H-shaped steel having a flange thickness of 40 mm or more.
Further, producing high strength steels through ordinary production procedures utilizing hot-rolling requires increasing the Ceq value of the steel. Increasing the Ceq value causes problems such as increased weld cracking and reduced toughness in the heat affected zone (hereinafter referred to as HAZ).
Moreover, gauge H steels require a rolling process wherein the rolling force of the mill per unit cross-sectional area of the rolled material is small. Therefore, rolling methods used for gauge H steels employ a low rolling reduction (rolling reduction/pass=1-10%) performed at a high temperature (950.degree. C. or more), which limits deformation resistance. However, satisfactory fine crystal grains cannot be obtained through this rolling method, and thus, satisfactory toughness cannot be achieved.
Some methods to which TMCP (ThermoMechanical Control Process) is applied are well-known as methods for producing a heavy-wall H-shaped steel having satisfactory strength, toughness and weldability.
For example, Japanese Patent Publication No. 56-35734 discloses a method for producing a gauge H steel with reinforced flanges, wherein a raw material is processed into a gauge H steel by hot rolling and then quenched to a temperature within a range of the Ar.sub.1 point to the Ms point from the external surface of the flange. Subsequently, the steel is air-cooled to form a fine low-temperature-transformed microstructure.
Further, Japanese Patent Publication No. 58-10442 discloses a method for producing a high tensile strength steel with excellent workability, wherein a heated steel is rolled at a low temperature within a range of 980.degree. C. to the Ar.sub.3 point with a rolling reduction of 30% or more to cause crystallization of ferrite, and then quenched to form a dual-phase microstructure of ferrite and martensite.
When applied to production of heavy-wall H-shaped steels, the methods taught in those publications cause many problems which could be attributed to quenching performed from the external surface of the flanges after hot rolling. For example, the strength and toughness in the thickness direction of the flanges are extremely irregular, and residual stress or distortion occur frequently.
Japanese Unexamined Patent Publication No. 3-191020 discloses a method for obtaining a gauge H steel having a low yield point and high tensile strength wherein a steel is mixed with Nb and V as elements for reinforcement, and is then subjected to a coarse rolling within a recrystallization temperature range at a rolling reduction of 30% or more. A subsequent finishing rolling is performed at about 800.degree.-850.degree. C., which is the Ar.sub.3 transformation point or higher.
This type of method utilizing Nb and comprising a rolling within a recrystallization temperature range and a rolling outside a recrystallization temperature range effectively produces gauge H steels of high strength and toughness. However, this method is inapplicable to the production of gauge H steels having a flange thickness of 40 mm or more for the same reasons discussed previously.
Furthermore, "Tetsu-to-Hagane" Vol.77, (1991), No. 1, p.171-! discloses characteristics of "As Rolled" steels produced with the addition of V and N and having a high strength. However, satisfactory strength and toughness could not be achieved when using the rolling conditions needed for producing heavy-wall H-shaped steels, namely, a low rolling reduction and a finishing temperature of 950.degree. C. or more.
Additionally, Japanese Unexamined Patent Publication No. 4-279248 discloses a method wherein a content of dissolved oxygen larger than usual is applied in the steelmaking step in order to generate an oxide of Ti, wherein the oxide serves as a core for crystallization of MnS, TiN and VN. In this method, Al deoxidation is not carried out, and crystallized MnS and other precipitates serve as cores for intransgranular ferrite formation to provide toughness for heavy-wall H-shaped steels.
The Publication uses a large content of dissolved oxygen while adding a Ti alloy and/or the like to the mold just before continuous casting in order to intentionally form fine Ti oxides. The Ti oxides thusly obtained serve as a core for crystallization of TiN and MnS, thereby resulting in fine ferrite which improves toughness. In addition, the steel described requires a large amount of labor in the steelmaking step and the continuous casting step since complicated processes must be performed to obtain the fine Ti oxide.